Field of the Invention
The present invention concerns a printing roller with a sleeve for a printing machine, e.g., a flexographic printing machine, including a rotary and longitudinal axis, where the printing roller is adapted for concentric mounting of the sleeve, and where the sleeve has a substantially internal cylindrical surface for contact with the printing roller, and wherein the sleeve has an external surface, the external surface being substantially cylindrical and adapted for mounting a plate.
Description of Related Art
The invention also concerns a sleeve for a printing roller for a printing machine, e.g., a flexographic printing machine, including a rotary and longitudinal axis, where the sleeve is adapted for concentric mounting on the printing roller, and where the sleeve has a substantially internal cylindrical surface for contact with the printing roller, and wherein the sleeve has an external surface, the external surface being substantially cylindrical and adapted for mounting a plate.
The invention further concerns a printing machine, e.g., a flexographic printing machine, which includes at least a doctor blade, a screen roller, a printing roller and a counterpressure roller, wherein the printing roller is adapted for mounting a plate, and use of a printing roller with a sleeve according to the invention for a printing machine.
It is commonly known that in connection with printing units a so-called doctor blade chamber is used from which ink/glue/varnish is applied to a roller, typically an anilox roller/screen roller. Moreover, ink is transferred from the screen roller to a plate/flexographic plate which is mounted on a printing roller/plate roller. On the printing roller, the plate is mounted either directly on the printing roller or on a so-called “sleeve” or “plate sleeve”. The term “sleeve” may well be translated into the Danish word “skørt” or “kappe”, but skilled persons working with these printing machines and their accessories however use the English word “sleeve” and is regarded as a technical term of art in the field. In the following, the term sleeve will be used to mean the covering item placed on the outside of the printing roller and on which is mounted an actual plate/flexographic plate.
By using sleeves, they, and thereby the mounted plate, can be rapidly replaced by simply dismounting the sleeve from the printing roller itself, and subsequently another sleeve with a different plate can be mounted. A sleeve is typically made of fiber composite in tubular form which is made by a suited process and with very great accuracy.
During operation of the printing machine, the respective rollers and the screen roller are bearing on the printing roller on which is mounted a plate, alternatively a sleeve with a plate. Between the printing roller and a counterpressure roller the medium on which printing is performed is conducted. The medium is typically in web form but may be constituted by single sheets of medium as well which are advanced in succession. This printing method is commonly called flexographic printing.
As mentioned, a plate extending longitudinally of the printing roller/sleeve is mounted on a printing roller or on a sleeve and along a greater or lesser part of the periphery of it and in the circumferential direction thereof. A plate is typically mounted with double adhesive tape between the plate and the surface of the printing roller/sleeve and with single adhesive tape at the leading edge of the plate. As the plate has a certain thickness, this means that when the leading edge of the plate strikes the screen roller, a jolt occurs, and when the screen roller encounters the trailing edge of the plate, a jolt occurs as well. The same kind of jolts are produced when the leading and trailing edges, respectively, encounter or leave the counterpressure roller. Since the printing roller and the other rollers are rotating at high speed, these jolts may more correctly be called vibrations that are transmitted throughout the printing machine, applying a high load on the printing machine, but also constituting a noise problem and thereby a challenge to the work environment. These jolts or vibrations are known by the term “bouncing” which is not desirable, but the problem has not hitherto been solved in connection with the use of sleeves on printing rollers.
Bouncing is a known problem with all flexographic printing machines, and entails, among others, the problem that when the plate strikes the leading edge of the screen roller, it bounces. This means that the printing roller and/or the screen roller are/is impacted by the jolt and experience(s) so large deflections that the rollers no longer are in contact with each other for some time/distance. This means that ink, glue or varnish is not transferred to the plate over a short length, typically 10 mm, or that an insufficient amount of ink, glue or varnish is transferred. The, e.g., the screen roller swings back and strikes the plate again, and then swinging once more with lack of transfer once again. Normally, bouncing occurs at least once and up to three times before the rollers do not have deflections of unfavorable magnitude. The same situation appears when the plate strikes the medium and the counterpressure roller. As mentioned, the result shows as a lack of ink, glue or varnish on the medium a short distance into the medium. The problem is particularly pronounced at the middle of respective rollers since it is here the rollers are vibrating the most. At the same time, these vibrations coming from bouncing will cause the rollers to hit their natural frequency more easily, further increasing the problem. An immediate solution to the problem with natural frequencies, and for that matter also of the problem with bouncing, can be changing the speed of the machine in order to apply ink, glue or varnish across the entire area of the medium.
Bouncing is of course entirely unwanted as it means depreciated quality or in the worst case that part of the products have to be discarded. At the same time, a reduced speed on the printing machine is unwanted as well since it reduces the capacity. An increased speed on the rollers may also be undesirable as by increased speed there may arise other challenges with regard to achieving the desired quality.
The problem of bouncing or vibrations produced by the jolts is particularly expressed when the application of varnish on the medium is involved, since a plate with a straight leading edge is frequently used here. However, the problem is very much present when applying glue and ink as well. That the plate has a straight leading edge striking the screen roller at once across the entire width of the plate, in this way actuating the screen roller away from the printing roller, means that bouncing is produced. Similarly, the straight trailing edge of the plate encounters the screen roller at once, whereby actuation caused by the thickness of the plate in direction away from the printing roller disappearing, also causing bouncing though in a lesser degree. Furthermore, the same occurs, in principle as also mentioned above, when the plate strikes and leaves the counterpressure roller. Here, however, there is not the same strong effect as by the screen roller which is due to the fact that the medium on which printing is performed in some cases act as a kind of shock absorber. In the cases where the medium is thin paper there is, however, very much the same problem with bouncing as between printing roller and screen roller. Thus, there are contributions from the screen roller as well as from the counterpressure roller contributing to the general effect called bouncing.
As mentioned, these circumstances are particularly problematic when the leading and/or trailing edge of the plate are/is straight and parallel with the rotary axis of the printing roller. When using a plate where the leading and/or trailing edge have/has a shape which is not parallel with the rotary axis of the printing roller, the problem is smaller, but still very present as the plate is mounted externally of the sleeve and thereby causes a thickening of the sleeve.
Even if a plate, in some cases, has a very small thickness of, e.g., a few tenths of a millimeter, the leading edge of the plate encounters the screen roller and leaves it again, irrespectively of its shape. A plate or flexographic plate can also be with a thickness of, e.g., one or more millimeters. This, in some cases modest, thickness and the fact that a certain contact pressure is to be maintained between plate/printing roller and the screen roller in order to ensure optimal application of ink, glue or varnish means that the jolts—bouncing—will arise.
In many cases are used printing rollers/sleeves with a circumference which is greater than the extension of a plate, and even several independent plates can be arranged on a printing roller/sleeve. In other words, this means by a rotation of a printing roller, one or more plate leading edges and one or more plate trailing edges will be present, all contributing to the unwanted bouncing vibrations.
None of the prior art methods for mounting plates on printing rollers or on sleeves for flexographic printing machines where the plate does not go all the way around the printing roller/sleeve are optimal, and they all have the drawback that they entail bouncing as mentioned above. There is therefore an acknowledged problem, but no solution to this problem.